The human eye is a highly evolved and complex sensory organ. It is composed of a cornea, or clear outer tissue which refracts light rays enroute to the pupil, an iris which controls the size of the pupil thus regulating the amount of light entering the eye, and a lens which focuses the incoming light through the vitreous fluid in the eye to the retina. The retina converts the incoming light to electrical energy that is transmitted through the brain to the occipital cortex resulting in a visual image. In a perfect eye, the light path from the cornea, through the lens and vitreous fluid to the retina is unobstructed. Any obstruction or loss of clarity within these structures, however, causes scattering or absorption of light rays resulting in diminished visual acuity. For example, the cornea can become damaged resulting in edema, scarring or abrasions, the lens is susceptible to oxidative damage, trauma and infection, and the vitreous fluid can become cloudy due to hemorrhage or inflammation.
As the body ages, the effects of oxidative damage caused by environmental exposure and endogenous free radical production accumulate resulting in a loss of lens flexibility and an accumulation of denatured proteins that slowly coagulate reducing lens transparency. The natural flexibility of the lens is essential for focusing light onto the retina by a process referred to as accommodation. Accommodation allows the eye to automatically adjust the field of vision for objects at different distances. A common condition known as presbyopia results when the cumulative effects of oxidative damage diminish this flexibility reducing near vision acuity. Presbyopia usually begins to occur in adults during their mid-forties; mild forms are treated with glasses or contact lenses.
Lenticular cataracts are a lens disorder resulting from protein coagulation and calcification. There are four common types of cataracts: senile cataracts associated with aging and oxidative stress; traumatic cataracts which develop after a foreign body enters the lens capsule or following intense exposure to ionizing radiation or infrared rays; complicated cataracts which are secondary to diseases such as diabetes mellitus or eye disorders such as detached retinas, glaucoma and retinitis pigmentosa; and toxic cataracts resulting from medicinal or chemical toxicity. Regardless of the cause, the disease results in impaired vision and can lead to blindness.
Treatment of severe lens disease requires the lens' surgical removal or phacoemulsification followed by irrigation and aspiration. However, without a lens, the eye is unable to focus incoming light on the retina. Consequently, artificial lenses must be used to restore vision. Three types of prosthetic lenses are available: cataract glasses, external contact lenses and intraocular lenses (IOLs). Cataract glasses have thick lenses, are uncomfortably heavy and cause vision artifacts such as central image magnification and side vision distortion. Contact lenses resolve many of the problems associated with cataract glasses, but require frequent cleaning, are difficult to handle (especially for elderly patients with symptoms of arthritis), and are not suited for persons who have restricted tear production. Intraocular lenses are used in the majority of cases to overcome the aforementioned difficulties associated with cataract glasses and contact lenses.
There are four primary IOL categories: non-deformable, foldable, expansible hydrogels and injectable. Early non-deformable IOL implants were rigid structures composed of acrylates and methacrylates requiring a large incision in the capsular sac and were not accommodative. This large incision resulted in protracted recovery time and considerable discomfort for the patient. In an effort to reduce recovery time and patient discomfort numerous small incision techniques and IOLs have been developed.
Early IOLs designed for small incision implantation were elastomeric compositions that could be rolled or folded, inserted into the capsular sac and then unfolded once inside. Occasionally, the fold of the IOL before insertion resulted in permanent deformation adversely affecting the implant's optical qualities. Further, while foldable IOLs overcame the need for the large incision non-deformable IOLs required, foldable IOLs still were not accommodative. Moreover, both non-deformable and foldable IOLs are susceptible to mechanical dislocation resulting in damage to the corneal endothelium.
Another approach to small incision IOL implantation uses an elastomeric polymer that becomes pliable when heated to body temperature or slightly above. Specifically, the IOL is made pliable and is deformed along at least one axis reducing its size for subsequent insertion through a small incision. The IOL is then cooled to retain the modified shape. The cooled IOL is inserted into the capsular sac and the natural body temperature warms the IOL at which point it returns to its original shape. The primary drawback to this type of thermoplastic IOL is the limited number of polymers that meet the exacting needs of this approach. Most polymers are composed of polymethylacyrlate which have solid-elastomeric transition temperatures above 100° C. Modifications of the polymer substrate require the use of plasticizers that may eventually leach into the eye causing harmful effects.
Dehydrated hydrogels have also been used with small incision techniques. Hydrogel IOLs are dehydrated before insertion and naturally rehydrated once inside the capsular sac. However, once fully rehydrated the polymer structure is notoriously weak due to the large amount of water absorbed. The typical dehydrated hydrogel's diameter will expand from 3 mm to 6 mm resulting in an IOL that is 85% water. At this water concentration the refractive index (RI) drops to about 1.36 which is unacceptable for an IOL. To achieve a RI between 1.405 to 1.410 a significantly thicker lens is required.
Modern acrylate IOLs generally possess excellent mechanical properties such as foldability, tear resistance and physical strength. Moreover acrylate IOLs are known to possess superior optical properties (transparency) and are also highly biocompatible. However, pure acrylic IOLs having the desired combination of mechanical, optical and biological properties may have unacceptable molecular response times such that the folded or compacted IOL may not unfold quickly enough to prevent post-insertion complications when inserted through a 3 mm or less incision. A pure acrylate IOL fabricated to have a molecular response time sufficient to minimize post-insertion complications can be extremely tacky and lack the desired mechanical strength. In this case, the resulting IOL may tear easily and/or the resulting self-tack can prevent unfolding. Thus pure acrylate IOLs are generally not suitable for incision sizes of 2 mm or less.
Pure silicone IOLs generally possess excellent mechanical, optical and biological properties similar to pure acrylate IOLS. Moreover, silicones also possess excellent molecular response times; in fact, the silicone IOLs are so responsive that when folded small enough to be inserted through a 3 mm or less incision, the stored energy can be so great that the IOL unfolds explosively damaging delicate eye tissues and structures. Consequently, pure silicone IOLs are not suitable for insertion through 2 mm or less surgical incisions.
Therefore there remains a need for IOLs that combine desirable mechanical, optical and biological properties with the ability to be compacted or folded into shapes or sizes that permit insertion through 2 mm or less, incisions without risking adverse post insertion complications.